Optical communication systems include devices, referred to as multiplexers, that route different wavelength signals from individual pathways into a common pathway and other devices, referred to as demultiplexers, that route the different wavelength signals from the common pathway back into the individual pathways. Often, the only difference between these devices is the direction of light travel through them.
The multiplexer/demultiplexer designs gaining widest acceptance are based on planar spectrographs containing phased arrays or reflective diffraction gratings. Within these devices, two optical mechanisms are used for routing the optical signals between the common and individual pathways--dispersion and focusing. Dispersion angularly distinguishes the different wavelength signals, and focusing converts the angularly distinguished signals into spatially distinguished signals.
For example, a focusing mechanism can be arranged to form discrete images of the common pathway in each wavelength of the different optical signals. The dispersing mechanism relatively displaces the images along a focal line by amounts that vary with the signal wavelengths. The individual pathways are arrayed along the focal line in positions aligned with the displaced images of the different wavelength signals. Thus, each different wavelength signal forms a discrete image of the common pathway in a different position along the focal line, and the individual pathways are located coincident with the image positions of the different wavelength signals.
The dispersing and focusing functions can be performed by phased arrays or reflective diffraction gratings. The phased arrays include a set of intermediate pathways (e.g., waveguides) that progressively vary in length to relatively incline wavefronts of different wavelength signals within a free spectral range. Confocal couplers connect the common and individual pathways to opposite ends of the intermediate pathways. Reflective diffraction gratings have a grating pattern that disperses the different wavelength signals at different diffraction angles and an overall shape that focuses the different wavelength signals in different positions along a so-called Rowland circle. Both the common and the individual pathways include inner ends located along the Rowland circle.
For small numbers of channels conveying different wavelength signals, the phased arrays work quite well but become more and more unwieldy with larger numbers of channels. Reflective diffraction gratings better accommodate large numbers of channels but are generally less efficient. One part of the problem is due to imperfections in the grating surface, and another part of the problem is due to polarization sensitivity.
Both problems can be reduced by enlarging grating pitch. However, the grating pitch is limited by other requirements of the design including wavelength dispersion, which decreases with increasing pitch. Additional dispersion can be obtained by increasing the diffraction order, but this decreases the free spectral range and increases efficiency variations between the different wavelength signals. Accordingly, efficiency improvements have been limited by competing design requirements.
The polarization of optical signals tends to fluctuate in optical communication systems, and the polarization sensitivity of the reflective diffraction gratings can cause large variations in transmission efficiency. Such large efficiency fluctuations lower signal-to-noise ratios of optical transmissions and require compensating amplification to avoid the loss of information.
U.S. Pat. No. 4,741,588 to Nicia et al. proposes to solve this problem in a multiplexer/demultiplexer by converting unpolarized light into substantially linearly polarized light before reaching a reflective diffraction grating. However, the conversion requires a special optical device formed by two prisms separated by a polarizing filter. The special device adds cost and complexity, and its use requires the focusing function to be performed by another optical element in advance of the reflective diffraction grating